German Publication DE-PS 919 657 discloses a device for deflecting asses through long ducts. In this device, uniform distribution of the gas through exit ports in the side walls of the long ducts is realized through the mounting of blades, but in a hook-like fashion, such that the blades extend into the duct at a right angle to the flow direction.
From DE-PS 919 657, a device for deflecting gasses that flow through long ducts is known. This device serves for uniform distribution to exit openings in the side walls of the ducts and comprises blades bent in a hook-like fashion, which extend into the duct at a right angle to the flow direction.
For hydrocarbon steam reformation, apparatus like the example shown in FIGS. 5 to 7, in different views and partial section, are often used. The prior art apparatus of FIGS. 5-7 has a substantially cuboid-shaped heating chamber 1, which is heated by a plurality of burners 2. The burners 2 are arranged proximate an upper surface of the heating chamber in rows in the roof of the cuboid, and their flame is directed vertically upward. Heat exchanger tubes 3 are arranged between the rows of burners on a vertical plane. A process medium for an endothermic chemical reaction can be run through these heat exchanger tubes 3. The process medium is heated primarily by the radiant heat produced during combustion, but partially by convection as well. To extract the combustion gasses from the heating chamber 1, waste gas ducts 4 are arranged on the bottom of the heating chamber 1. The waste gas ducts 4, are closed on top. The waste gas ducts 4 are arranged parallel to the rows of heat exchanger tubes 3 and the rows of burners and have a substantially rectangular cross-section. The waste gas ducts 4 are made of a refractory-grade material (e.g., built of refractory-grade bricks). Numerous openings 5 are distributed along the axial length of the waste gas ducts 4 in the lower part of their side walls. Through these openings 5, the waste gas can make its way from the heating chamber 1 into the interior of the waste gas ducts 4. The waste gas then flows through the waste gas ducts 4 laterally out of the heating chamber 1 and enters a collection ducts, into which all waste ducts 4 open. The waste gas is fed through the collection duct to a means for using the residual heat still contained in the waste gas, and finally discharged into the open air.
In the practical operation of the prior art apparatus, it has been found that heat transfer to the heat exchanger tubes is not uniform. This is reflected by the evidence found during the maintenance of such prior art apparatus. If some of the heat exchanger tubes are exposed to greater heat, these tubes age more rapidly and must be replaced sooner. In this situation, the choice is between either replacing all of the heat exchanger tubes, including those that do not yet need to be replaced, or exchanging only some of the tubes. The latter choice results in more frequent interruptions of operation for maintenance and repair and the former is more expensive in that some of the heat exchanger tubes are replaced before the end of their useful life.
Because the heat exchanger tubes are heated largely by radiant heat, it is very important that the tubes be exposed to the same conditions with respect to the effects of the heat radiation source. When the waste gas ducts have a rectangular cross-section, this happens as a matter of principle. FIG. 8, which shows a sliced section through a heating chamber 1, schematically illustrates such a rectangular duct in greater detail. The openings 5 in the side walls of the waste gas duct 4 for the passage of waste gasses are arranged along a bottom of waste gas duct 4. Another known form for a waste gas duct 4a is shown in FIG. 9. This waste gas duct 4a also has a rectangular cross-section, but its cross-section changes continuously in the direction of the waste gas discharge (collection duct 8). The step-like roof of the waste gas duct 4a is inclined in such a way that the cross-section continuously grows larger in the direction of the collection duct 8. This design of the waste gas duct 4a is intended to ensure that waste gas is extracted uniformly over the axial length of the waste gas duct. This does not happen in the case of the cuboid-shaped waste gas duct 4 in FIG. 8, due to pressure loss. However, the shape of the waste gas duct 4a provides no effective improvements in this respect. In both cases, therefore, uneven heating of the heat exchanger tubes occurs. If it were possible to ensure uniform distribution during the transfer of heat to the heat exchanger tubes, then the heat exchanger tubes could be designed with thinner walls, because the higher wall temperatures that occur in some tubes when there is uneven heat distribution would no longer have to be taken into account. Accordingly, more extreme operating conditions could then be applied or the useful life of the heat exchanger tubes could be extended during normal operation.